The present invention relates to a method and a circuit for the reduction of the temperature dependency of the oscillting properties of a LC oscillator.
Oscillators which make use of a LC-oscillating circuit as a frequency determning element are utilized in inductive proximity switches. When the driving blade of an inductive proximity switch approaches the coil of the LC-oscillating circuit, it damps the quality of the latter which is utilized by an evaluation circuit for producing an output signal.
The principal disadvantage of known inductive proximity switches is their relatively small switching distance. The switching distance can only be increased with a concomitant adverse effect on the high temperature dependency of this switching distance.
A very elegant solution to this problem has been described in U.S. Pat. No. 4 509 023. This solution permits in practice, elimination of the principal cause of the temperature dependency, that is the temperature coefficient of the copper resistance.
A disadvantage of the method of U.S. Pat. No. 4 509 023 is that only the loss relating to the temperature coefficient of the copper resistance of the coil is cancelled. However, a LC-oscillating circuit still has a number of other sources of losses which operate to oppose any further increase of the temperature independency of the oscillator.